Pterygotioidea

Restorations of slimonid Slimonia (top) and derived pterygotid Pterygotus (bottom).

Pterygotioid eurypterids, whose fossils are recovered in deposits ranging in age from the Early Silurian[3] to the Late Devonian,[4] can be distinguished from all other eurypterine eurypterids by the placement of their eyes, being located near or in some cases on the margin of the carapace (the "head" plate).[3] Pterygotioids ranged in size from small eurypterids, the smallest being Hughmilleria wangi at just 6 centimetres (2.4 in) in length, to the largest arthropods to ever live, the largest being Jaekelopterus rhenaniae which might have reached lengths of 2.6 metres (8.5 ft).[5][1]

Like all other chelicerates, and other arthropods in general, pterygotioid eurypterids possessed segmented bodies and jointed appendages (limbs) covered in a cuticle composed of proteins and chitin. In the Pterygotidae, the outer surface of the exoskeleton was covered in a scale-like ornamentation but it was smooth within Slimonidae and Hughmilleriidae.[6]

The chelicerate body is divided into two tagmata (sections); the frontal prosoma (head) and posterior opisthosoma (abdomen). The appendages were attached to the prosoma, and were characterized in pterygotids and slimonids by being small and slender and lacking spines.[6] In contrast, the more basal Hughmilleriids did possess spines on their appendages.[7]

In derived members of the group, Slimonidae and Pterygotidae, the telson (the posteriormost segment of the body) was expanded and flattened, often with a spike protruding from its end.[8] The telsons of the Hughmilleriidae were not flattened, instead being lanceolate (in the shape of a lance or spike) and similar to those of more primitive eurypterids such as Eurypterus.[9]

Like other chelicerates, pterygotioids possessed chelicerae. These appendages are the only ones that appear before the mouth and take the form of small pincers used to feed in most eurypterid groups. This function is retained in the more basal Hughmilleriidae and Slimonidae, but pterygotid chelicerae were large and long with strong and well developed teeth on specialized chelae (claws).[3]

Fossils described by John William Salter as belonging to "Pterygotus problematicus" in 1852. This species is now considered a nomen vanum.

The first pterygotioid fossils to be uncovered were those of the type genus, Pterygotus. Louis Agassiz, a Swiss-American biologist and geologist, described the fossils in 1839 and named the genus Pterygotus, meaning "winged one". Agassiz mistakenly believed that the fossils were the remains of a large fish.[10] Agassiz first recognized the true nature of the fossils as arthropod remains five years later in 1844 after having examined more complete fossils recovered in the Old Red Sandstone of Scotland.[11][12]

In 1856, the species Pterygotus acuminata was named by John William Salter. The fossils referred to this species, recovered from Lesmahagow, Scotland, were soon realized to be distinct from other species of Pterygotus (such as the type species P. anglicus) and that same year geologist David Page erected a new genus to contain the species. The new genus, Slimonia, could be differentiated from other known species of Pterygotus most apparently by the lack of large cheliceral claws, otherwise a defining characteristic of Pterygotus.[13]

In 1903, the genus Hughmilleria was created based on fossils discovered in the Pittsford Shale Member of the Vernon Formation. Its describer, the American geologist Clifton J. Sarle, considered the genus to represent an intermediate form between the more basal Eurypterus and the derived Pterygotus but did not assign Hughmilleria to any particular family.[14]

The family Pterygotidae was erected in 1912 by John Mason Clarke and Rudolf Ruedemann to constitute a group for the genera Pterygotus, Slimonia, Hastimima and Hughmilleria.[15] Pterygotus had also been designated as containing two subgenera; Pterygotus (Curviramus) and Pterygotus (Erettopterus), but Erettopterus would later be raised to its own genus.[12] American paleontologist Erik N. Kjellesvig-Waering emended the Pterygotidae in 1951, referring the genera Hastimima, Hughmilleria and Slimonia, and the newly named Grossopterus, to their own family within the Eurypteracea, Hughmilleriidae, leaving Pterygotus as the sole pterygotid genus.[12]

In 1955, Norwegian paleontologist and geologist Leif Størmer considered the pterygotid clade to represent a family within the eurypterid superfamily "Eurypteracea".[6] In 1962, Russian paleontologist Nestor Ivanovich Novojilov raised the Eurypteracea and Pterygotidae to subordinal and superfamily status, Eurypteracea becoming the suborder Eurypterina and creating the superfamily Pterygotioidea, containing Hughmilleriidae and Pterygotidae.[16][8] The same year, Novojilov also reclassified Slimonia into a pterygotioid family of its own as it was considered distinct enough from other hughmilleriids to warrant a separate family, the Slimonidae.[11]

Eurypterids are most commonly recovered from fossil deposits in Scandinavia, Eastern Europe, Britain and North America. During the Silurian and Devonian periods, when pterygotioids were alive, these regions and continents were part of the continents Baltica (Scandinavia and Eastern Europe), Avalonia (Germany, Britain, parts of eastern North America) and Laurentia (most of eastern continental North America). It is around these continents, and the Rheno-Hercynian Terrane (western and central Europe), that pterygotioids are the most common.[2]

Pterygotioid fossils have also been recovered from other parts of the world where fossils of other eurypterid groups are absent, including Australia, Morocco, Libya, Florida, Saudi Arabia, China, Paris, South America, Bohemia and Siberia, which indicates that the group had spread significantly during their 70 million year existence. Pterygotioids appear to have been relatively abundant throughout the Silurian and Devonian world, with unusually (in terms of eurypterids, most groups being absent entirely) large numbers recovered from the ancient continent of Gondwana (composed of Africa, India, South America, Australia and Antarctica).[2]

The earliest known pterygotioids, belonging to the genus Hughmilleria, are from the Llandovery epoch of the Silurian and appear around the same time in Laurentia, Scotland and South China. With the group already being widespread at this seemingly early stage in their evolution, it is difficult to pinpoint their exact geographical point of origin. The closest relatives and sister clade of the pterygotioids, the superfamily Adelophthalmoidea, originated in Laurentia. Modern researchers assume that the case would be the same with the pterygotioids, which are thought to have originated within, or in close proximity to, Laurentia.[2]

It remains unclear whether pterygotioids were capable of crossing oceans on a regular basis or if species recovered outside of Laurentia, Baltica and Avalonia represent isolated occurrences. Pterygotioids were, like the related adelophthalmoids, excellent swimmers which might help explain the intercontinental dispersal patterns and wide-ranging distribution seen in both superfamilies.[2]

• List of eurypterid genera
• Timeline of eurypterid research